2001 Buick Lesabre Fuel Gage

I had my car since August 2005 and had this problem when I first bought but haven't had time to get it look at. My gas meter in the dashboard doesn't read the fuel in the gas tank correctly. It will show how much gas I have when I first start the car up then a minute after that it will move slowly up or down. Sometimes the gas hand reads full when it really near empty. I wanted to know is it the flow device in the gas tank or the gage in the dashboard?